Separation of metals



United States Patent SEPARATION OF METALS Albert L. Jaife, Philadelphia, Pa.

No Drawing. Application October 25, 1956 Serial No. 618,192

11 Claims. (CI. 41-42 This invention relates to the art of separating unlike metals. More particularly it relates to the art of separating relatively low melting point metals and alloys from copper and copper-base alloys. It is primarily concerned with the removal of tin, lead, and tin-lead alloys from copper and brass articles of manufacture. The removal of zinc and antimony and their alloys from copper or, copper alloyed articles is also contemplated.

The principal present method for removing solder (a tin-lead alloy) from copper or copper-alloy bases such as, for example, automobile radiators, has been sweating the radiator in ovens'and vibrating to remove the solder. Such a method is disclosed in US. Patent 1,826,755. From a view point of reclaiming the copper, the sweating process has disadvantages. It is difiicult to thoroughly remove all the solder materiaL- If the temperature is high enough to remedy this, the danger of alloying the tin component with the copper is encountered. This makes the copper brittle and also decreases its scrap value. Furthermore, oxides of both the copper base and the low melting point metals are produced resulting in a lower grade copper scrap.

Where, throughout this specification and claims, the word copper or copper-based is used, it is meant toinclude all copper base alloys such as brass (85% copper, 15% zinc) or bronze (copper and tin) where a major portion of the alloy is copper.

It is an object of this invention to provide a method for the removal of relatively low melting point metals and alloys from copper and copper alloy bases.

It is an object of this invention to remove tin, lead, solder and Babbitt metal from copper and brass articles.

It is another object of this invention to remove relatively low melting point metals from copper and copper alloy bases by a method comprising immersion in a bath of fused sodium hydroxide and sodium hydride, producing a high-grade copper scrap.

\ A still further object of this invention is the provision of a series of steps in the removal of relatively low melting point metals and alloys from copper and copper alloy bases comprising pre-heating with indirect heat, sodium hydride contact, quenching and rinsing.

These and other objects will be made clear in the following detailed description.

Some of the copper and copper alloy articles from which the low melting metals may be removed are tinplated copper wire, light copper spouting (light gauge roofing materials), sheet copper, painted and plated yellow brass stock, nickel-plated brass, plated or soldered copper tubing, electrotype shells (copper base With redantimony coating), honeycomb radiators, nickel-plated red brass spigots, valves and pipes, and babbitted bearings. These articles are not to be construed as exhaustive of this inventions uses but are merely illustrative of some of the many common commercial uses.

, Copper wire plated with tin may be stripped of its tin by the following process according to this invention.

The copper wire, without any prior heating or other treatment is immersed in a bath of sodium hydride and sodium hydroxide. The proportion of sodium hydride in the bath should be at least 1.0% and preferably at least 1.6%-2.0%. The proportion of NaH may be as high as 20.0%, but about 2.0% is workable and commercially feasible. A NaH concentration of only 0.5% has been found unsatisfactory. The temperature of the bath should be between about 675 degrees and 775 degrees F., preferably in the range of 700 degrees to 750 degrees F. The temperature and concentration conditions of the bath are the same as in the prior art disclosures for pickling or de-scaling ferrous materials. The tin-plated copper wire is immersed in the bath for a period of approximately 10 to 15 minutes. This time may vary depending on the conditions, such as the thickness of the plate.

' When withdrawn and washed or rinsed with hot water again successful.

to remove the sodium hydroxide coating or phlegm, the copper was found to be clean and to be of 99.98% purity.

It has been known to use sodium hydride in a fused bath of sodium hydroxide to remove oxide scale from steel, iron, and steel alloys such as stainless steel. Such a process is disclosed for example, in U.S. Patent No. 2,377,867. The materials producing the sodium hydroxide-sodium hydride bath are marketed by several companies. For example, the El. du Pont de Nemours and Co., Inc. supplies the appropriate chemicals and information as to the requisite apparatus. The Du Pont process is described as being applicable to the removal of scale from many metals and alloys such as nickel, Monel, Stellite, titanium, and cobalt bearing alloys. Other producers of hydride descaling materials are Metal Hydrides, Inc., Beverly, Mass., and U8. Industrial Chemicals Co., N.Y., N.Y.

The sodiurn hydride-sodium hydroxide bath is generally prepared as follows: A container of sodium hydride, equipped with heating elements, is heated to a temperature to which the sodium hydroxide fuses. Associated with the container is a generator box, into which metallic sodium is charged and into which hydrogen is bubbled. Sodium hydride is produced and mixes into the sodium hydroxide bath. The concept of the production of this hydride bath is not deemed part of the present invention and the use of other existing methods and apparatus for NaH production is contemplated. The apparatus and method for hydride bath production is well known in the art and it is not deemed necessary to describe this method and apparatus in detail in this disclosure. Methods and apparatus for the provision of sodium hydride baths are disclosed, for example, in U.S. Patent 2,353,062, the booklet entitled Du Pont Sodium Hydride De-scaling Process, published by Du Pont Company or a Technical Bulletin 507C published by Metal Hydrides, Inc., Beverly, Mass.

Example 1 Copper wire coated with tin was placed into a fused bath of 1.8% NaH in NaOH. The bath was maintained at a temperature of 750 degrees F., without prior treat ment. After 10 minutes in the bath the wire was with drawn and found to be completely clean of tin. The treated copper was 99.98% pure.

Example 2 200 pounds of tin coated copper wire without prior treatment was immersed in a fused bath of 1.6% NaOH at 720 degrees F. After 10 minutes the wire was withdrawn, flushed, or rinsed with hot water to remove the coating or phlegm of NaOH. The purity of this copper wire was found to be 99.9%. This process was then repeated in the same fused bath and was found to be asarsss It was feared in Examples 1 and 2 that the tin would go into solution in the NaOH and therefore recoat subsequently treated samples but this was found not to be the case. It is not completely understood why this process should remove the tin plate so efficiently. Merely exposing tin-plated copper wire to indirect heat of temperature equal to that of Examples 1 and 2 does not produce completely de-tinned copper wire.

Where copper or brass articles totally or partially coated with solder are to be de-soldered it has been found that a preliminary step to the NaI-I bath is desirable. The article should first be subjected to indirect heat of approximately 900 degrees F. By indirect heat is meant heat supplied electrically or in any other manner wherein the work is not contacted by direct flame. The work is preheated at this temperature for a period of time which will vary depending on the character of the article to be treated (such as the thickness of solder), but it has been found that for an article such as a copper honeycomb radiator, one hour pre-heating is satisfactory.

After the pre-heating stage the work piece is transferred immediately to a bath of fused NaOH and NaH. The temperature and NaH concentration of the bath is the same as described above in connection with de-tinning copper wire.

The period of immersion in the fused bath may vary depending on the conditions (as the amount of material to be removed, the temperature of the bath, and the concentration of the bath). It has been found that for pieces such as soldered radiators a period of to minutes is satisfactory.

Example 3 A copper radiator, having soldered connections, was pre-heated in indirect, electric oven heat at 900 degrees F. for one hour. The radiator was then immersed in a fused bath of NaOH with 2.0% Nal-I at a temperature of 750 degrees F. it was maintained in this bath for 20 minutes. The piece was then withdrawn, shaken, water quenched and hot water rinsed. The piece was then pickled for 10 minutes in 10% NHO at room temperature, rinsed, and air dried.

Example 4 A cooper radiator having soldered connections was preheated in indirect, electric oven heat at 900 degrees F. for one hour. The radiator was then immediately immersed in a fused bath of NaOH with 2.0% NaH at a temperature of 750 degrees F. It was maintained in this bath for 20 minutes. The piece Was withdrawn, shaken, water quenched, and hot water rinsed. The piece was then pickled for 10 minutes in 10% HNO at room temperature, rinsed, and air dried.

The copper reclaimed by means of Examples 3 and 4 tested to a purity of 99.90%. Substantially the same treatment as described in Examples 3 and 4 was performed on a work piece made of brass (approximately 85% copper, 15% zinc). After treatment, the brass piece also showed as complete freedom from lead and tin as did the copper work piece. The steps of quenching, rinsing, and acid pickling are not alleged to be novel per se. Such steps have often been performed in connection with sodium hydride de-scaling and in de-scaling in general. The acid treatment step brightens the reclaimed copper or copper alloy material, but is not an essential step to the inventive process herein disclosed.

In addition to soldered metal coatings, Babbitt metal (alloys of antimony, lead and thin) coatings may be successfully treated with this process. This class of alloys is used, for example, in bushings, bearings, and electrotype shells. It has been found that pre-heating electrotype shells with indirect heat at temperatures from 700 degrees to 900 degrees F. will result in the sweating of much of the white metal alloy from the copper shell, leaving a substantial remainder. Immediate immersion of the shell into a fused NaOH-NaH bath of the same range of temperature, strength, and times as in the preceding examples results in the complete removal of the white metals from the copper piece. if

Attempts to completely remove lead, tin, antimony and various alloys from copper or copper alloy bases by heating alone are not completely satisfactory. If the heating is of high enough temperature and of long enough duration to effectively remove all the low melting metal, undesirable results may occur. The low melting metals may form a surface alloy with the copper thus greatly reducing the commercial value thereof. Another possibility is the production of oxides of copper on the surface, which also greatly down grades the commercial value, resulting in burnt copper.

If the heating conditions are moderated to avoid these problems, the low melting metals are not completely removed from the base. They often melt out into a thin adherent film on the base metal. In the aspects of the present invention wherein a pre-heating step is used, the material need not be heated to the point of danger of alloying or burning. Pre-heating to a point at which some coating still remains on the base and then relying on the hydride bath to remove the remainder of the coating produces a clean copper. As indicated above, for example, a temperature of about 900 degrees F. is satisfactory for soldered copper articles. The exact pre-heat temperature is not highly critical, but the temperature range is important. A pre-heat temperature of 600 degrees F. was found to be wholly unsatisfactory; a temperature of 900 degrees F. was completely satisfactory in the case of solder. For Babbitt metal, a range of about 700 degrees F. to 900 degrees F. is satisfactory.

A reason that indirect heating is preferred for the preheating step is that direct heat may produce hot spots which tend to oxidize or alloy with the base material.

It is not completely understood why the present preheating and hydride bath process is so successful. It has been suggested that in the pre-heating step, some low melting material is simply melted away, and that thereafter a film or other formation of the coating metal remains adherent to the copper or copper alloy base, and is retained or dammed by a crust of low melting alloy oxide which prevents flow of the inner film. It is thought that upon immersion in the hydride bath, the oxide crust may be reduced, and since high temperature is present in the bath, the remaining film is free to flush away.

It is understood, of course, that this is merely a hypothesis. In the case of immersion of tin-coated articles without pre-heating, another explanation must be found. It may be that the hydride bath is effective when the coating is thin, either initially as in the case of tinned wire, or because of the pro-heating (sweating) step.

The term coating is understood to mean complete or partial covering of the copper or copper alloy base metal by a relatively low melting-point metal or alloy.

While sodium hydride is commercially feasible, other metal hydrides, as lithium hydride and potassium hydride may also act under conditions of temperature and concentration suitable to their known chemical properties.

The term low melting-point metals is understood to mean the metals and their alloys.

The scope of this invention is to be determined by the appended claims and not by the specific examples and illustrations set forth in the description.

I claim:

1. The method of removing coatings of relatively low melting-point metals from a copper article comprising immersing said article in a fused bath of sodium hydroxide and at least 1.00% but not more than about 20.00% sodium hydride until said relatively low meltingpoint metal coatings are separated from said article, and then removing said article from said bath, whereby a high grade copper scrap is produced.

2. The method as defined in claim 1 wherein the coating is selected from the group consisting of relatively low melting point metals and their alloys and the article is selected from the group consisting of copper and copper alloys.

3. The method as defined in claim 1 wherein the coating is selected from the group consisting of lead, tin, antimony and their alloys and the article is selected from the group consisting of copper and copper alloys.

4. The method as defined in claim 1 wherein the temperature of said bath is between about 675 degrees F. and 775 degrees F.

5. The method as defined in claim 1 wherein said fused bath contains from about 1.6% to 2.0% of sodium hydride and the temperature of said bath is between about 700 degrees F. and 750 degrees F.

6. The use of a fused bath comprising a major proportion of sodium hydroxide and containing a metal hydride for the purpose of removing a coating of relatively low melting-point metals from a copper article comprising immersing said article in said fused bath until said relatively low melting-point metals are removed from said copper article, whereby a high grade copper scrap is produced.

7. The use as defined in claim 6 wherein the metal hydride is sodium hydride constituting at least about 1.00% and no more than about 20.00% of the said bath.

8. The method of completely removing tin from tin plated copper wire comprising immersing said plated wire in a fused bath of sodium hydroxide and sodium hydride wherein said bath contains at least about 1.00% of sodium hydride but no more than about 20.00% and the temperature of said bath is between about 700 F. and 750 F. without any prior heating of said wire, maintaining said wire in said bath until the tin has separated from the wire, and then removing the wire from the bath and rinsing the wire with hot water, whereby copper of substantially perfect purity is obtained.

9. The method of removing solder from a copper article comprising pre-heating said article in indirect heat at a temperature of about 900 degrees F., immersing said article in a fused bath of sodium hydroxide and sodium hydride until said solder is separated from said article, wherein said bath contains at least 1.00% but no more than 20.00% of sodium hydride and the temperature of said bath is between about 675 F. and 775 F. withdrawing said article from said bath and rinsing with hot water.

10. The method of completely removing an alloy comprising lead, tin and antimony from a copper article comprising pre-ieating said article at a temperature of between about 700 degrees F. and about 900 degrees F. in indirect heat, immersing said article in a fused bath of sodium hydroxide and sodium hydride wherein the said bath contains at least 1.00% but no more than 20.00% of sodium hydride and the temperature of said bath is between 675 F. and 775 F. until said alloy is separated from said article, and withdrawing said article from said bath.

11. The method of removing a coating of at least one material of the group consisting of lead and tin from the surface of a copper-base metal object, comprising the steps of immersing said copper-base metal object in a bath consisting essentially of fused sodium hydroxide and sodium hydride at a temperature of at least about 750 F., maintaining said object in said bath until said coating is separated from said object, and removing said object from said bath.

References Cited in the file of this patent UNITED STATES PATENTS 2,377,876 Gilbert June 12, 1945 2,717,845 Carter Sept. 13, 1955 FOREIGN PATENTS 232,257 Great Britain Dec. 10, 1925 466,661 Great Britain May 26, 1937 

1. THE METHOD OF REMOVING COATINGS OF RELATIVELY LOW MELTING-POINT METALS FROM A COPPER ARRTICLE COMPRISING IMMERSING SAID ARTICEL IN A FUSED BATH OF SODIUM HYDROXIDE AND AT LEAST 1.00% BUT NOT MORE THAN ABOUT 20.00% SODIUM HYDRIDE UNTIL SAID RELATIVELY LOW MELTING-POINT METAL COATINGS ARE SEPARATED FROM SAID ARTICLE, AND THEN REMOVING SAID ARTICLE FROM SAID BATH, WHEREBY A HIGH GRADE COPPER SCRAP IS PRODUCED. 